The high energy demand in the world has subjected oil prices to wild swings but the demand for oil continues unabated. It is also clear from recent reports that the energy industry has to continue increasing the supply of hydrocarbon fuels to meet the global energy demand. However, an offshore hydrocarbon field will only be exploited if the field can produce enough net income to make it worth developing at a given time; dependent upon a combination of technical, commercial, regulatory, production sharing terms and also an oil & gas company's own Internal Rate of Return. Most of the fields that have been developed to date have been based on the “satellite principle,” which means that existing pipeline transportation infrastructure and production facilities in the vicinity of the identified field are used so that the development costs are significantly reduced. The remaining unexploited fields are often located in remote locations with little or no infrastructure and of a size or nature that often make it impossible to predict with certainty the amount or composition of recoverable hydrocarbon in place. These fields are often referred to as small, marginal, unconventional reservoirs or stranded assets.
The wild swings in oil prices however brings with it new challenges. Development costs have been pushed to new highs. Competition for the same resources for, e.g., skilled manpower, specialized plant and equipment and space in fabrication yards have also resulted in resource constraints.
Consequently platforms have been over or under designed resulting in technicians, tools and equipment transported to site to carry out costly modifications.
Conventional offshore platforms are built from components that are transported separately from fabrication sites to offshore installation sites where they are put together utilizing barge mounted heavy lift cranes and/or jack-up drilling rig mounted derricks.
Due to the shortage of such crane barges and jack-up rigs, the mobilization or demobilization costs and day rates for these units have increased. Installation of platforms to extract hydrocarbons at small and marginal fields is no longer economically viable mainly due to this escalation in costs.
There is therefore an urgent requirement to considerably reduce the costs of these marginal field developments and consequently make these developments economically viable. Many concepts have already been developed and are being offered by the industry.
This led to the invention of self-installing platforms called mobile offshore production unit which can easily be re-locatable without the need for a derrick barge or a jack-up drilling rig. The mobile offshore production unit is used adjacent to a wellhead platform that supports drilling operation. Hydrocarbons extracted via the wellhead platform are sent to the mobile offshore production platform for separation and further conditioning before being returned to the wellhead platform for onward transportation to a pipeline network or a Floating Storage and Offloading (FSO) vessel.
The mobile offshore production unit can only be operational where a pre-installed wellhead platform with risers linked to a pipeline network or an FSO is present. For marginal fields and medium sized fields in remote locations where a pipeline network is non-existent, the high costs associated with the installation and de-installation of the wellhead platform and an FSO will not make the project economically viable.
Therefore, there is a need for the mobile offshore production unit to be made versatile for use at small and marginal fields with solutions to overcome uncertainty and high costs associated with the installation of wellhead platforms for drilling and FSO for storage.
Furthermore, these wellhead platforms are constructed based on assumptions on the likely outcomes of ultimate hydrocarbon recovery. These outcomes are based on seismic data and/or exploration wells drilled at the location. This method has often resulted in over design and sub optimized platforms resulting in unnecessary capital expenditure for the field owner/operator. It is widely acknowledged that economics of exploiting stranded assets are easily affected by changes in basic economic conditions such as capital expenditure, time to first oil, operating costs, production levels, recoverable reserves and abandonment costs which can have a major effect on the profitability of the venture. If a field is marginal because of the uncertainty over the level of reserves, a period of exploration often referred to as extended well test will give additional reservoir information and will reduce uncertainty thereby leading to improved decision making. There is therefore an urgent need for an operator or field owner to exploit these so called stranded assets in an incremental, optimal and cost effective manner.
Several methods have been developed for the installation of wellhead platforms without using crane barges and drilling rigs. One such method is the Suction-piled Stacked Frame (SSF) platform as described in the “Proceedings of the Eleventh (2001) International Offshore and Polar Engineering Conference, Stavanger, Norway, Jun. 17-22, 2001, a purpose designed satellite wellhead platform. The following is an extract from a paper presented during the conference:
“The attractiveness of the SSF platform is essentially based on its cost-effectiveness compared with existing marginal platform concepts, whereby the main cost differentiators are the efficient use of materials and the installation method. The SSF platform consists of three conductors that support a small deck, the export riser and a ladder arrangement for safe access from a boat. The base of the structure comprises a frame, which incorporates suction cans and conductor guides. The conductors are simultaneously used as jacket legs and they are positioned approximately 7 meters from each other. They are braced by three frames that are positioned at the appropriate elevation to give adequate structural strength. The frames are being fixed to the conductors by means of grouting.
Apart from the drilling and jacket leg function, the conductors also form part of the foundation. However, depending on water depth and the environmental loading, the three conductors will in many cases not have sufficient bearing capacity on their own and hence additional suction cans are added to make up the SSF platform foundation. The main function of the suction cans is to carry the base shear, but they carry part of the vertical loads, caused by the overturning moment, as well. The suction cans are connected to the lowest stacked frame and they are positioned outside the footprint of the conductors. The upper stacked frame, apart from providing stability and stiffness to the structure, simultaneously serves as the topside deck.”
The SSF and similar wellhead platform installation concepts are suitable for minimum facilities developments with limited number of wells (up to 6) and minimal topsides facilities (up to 150 MT) as stated in the above paper, allowing in most cases only primary recovery of hydrocarbons. More well slots are required for secondary recovery via water injection, gas lift, etc. to maximize recovery. Larger wellhead platforms offering the flexibility to add conductors and wells as the field develops have become a necessity.
Therefore there is a need for a method to install wellhead platform with required number of wells that eliminates steps of separately installing the wellhead platform using crane barges and/or jack-up drilling rigs resulting in an optimal configuration.